This invention relates to expansion seal assemblies and, more particularly, to such an assembly for sealingly connecting components of fluidized bed reactors passing high temperature combustion gases and particulate material.
Reactors, such as combustors, steam generators and the like, utilizing fluidized beds as their primary source of heat generation, are well known. In these arrangements, air is passed into the furnace section of the reactor and through a bed of particulate material contained therein which includes a mixture of a fossil fuel, such as coal, and an adsorbent, such as limestone, which is used to adsorb the sulfur generated as a result of combustion of the coal. The air fluidizes the bed and promotes the combustion of the fuel. The hot combustion gases are passed to a heat recovery area where their heat is removed to perform work, such as to drive a steam turbine.
The most typical fluidized bed combustion system is commonly referred to as a "bubbling" fluidized bed in which a dense bed of the particulate material is supported by an air distribution plate to which the combustion supporting air is introduced through a plurality of perforations, causing the particulate material to expand and take on a suspended, or fluidized, state.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a fluidized bed reactor was developed utilizing an expanded and elutriating fluidized bed commonly referred to as a "circulating" fluidized bed. In these arrangements, the size of the particulate material is decreased and/or the mean air velocity is increased when compared to the bubbling bed, so that the bed surface becomes more diffused and the entrainment of particulate material from the bed is increased. According to this process, in the lower portion of the furnace section, fluidized bed densities are attained which are well below those typical of bubbling fluidized beds, whereas the upper portion of the furnace section becomes loaded with entrained particulate material, or solids, to a much greater extent than in bubbling fluidized beds. This increased solids entrainment in the upper portion of the furnace section results in a high solids throughput which requires a high solids recycle rate. Reactors having high solids recycle rates require separators to separate the entrained particulate material from the hot combustion gases before the gases pass through the heat recovery area in order to reduce erosion of the heat recovery surfaces in the heat recovery area.
U.S. Pat. Nos. 4,809,623 and 4,809,625, assigned to the same assignee as the present application, disclose a fluidized bed reactor in which a dense, or bubbling, fluidized bed is maintained in the lower portion of the furnace section, while the bed is otherwise operated as a circulating fluidized bed. The design is such that advantages of both a bubbling bed and a circulating bed are obtained, not the least significant advantage being the ability to utilize particulate fuel material extending over a greater range of particulate sizes.
In these arrangements, passages are formed between the steam generating system components for allowing the combustion gases and particulate material to flow from one component to the next. These passages are embodied in conduits interconnecting the system components, or, when the distance between adjoining components is short, in the integrally connected walls which form the component outlets and inlets. These connections are complex structures since, in normal operation, the materials being passed between the various components of a steam generating system can have temperatures in excess of 2000.degree. F. and pressures either below or above atmospheric.
Due to the extreme temperatures involved, the seals forming the connections between the steam generating system components and/or conduits must accommodate relative thermal expansion and contraction between the connected structures, especially when the components are anchored to the structural steel supporting the system. Such seals are relatively expensive since the relative expansion and contraction occurs in two lateral directions as well as in the axial direction.
Known seal designs are subject to failure due to erosion and thermal degradation caused by the impingement of the heated particulate material passing through the passages. The cost of replacing failed seal assemblies and the accompanying lost operating time is great.
The design of seal assemblies is further complicated due to the pressure differentials that can exist between the flow within the steam generating system and the outer atmosphere. The seal must be designed to prevent air infiltration when the steam generating system is operated under suction but must also be designed to prevent gas leakage when the system is run at pressures above atmospheric.